Planar antennas are antennas which are not matched to a defined impedance but require a matching network for maximum power transmission.
A connection of a planar antenna to an associated impedance matching circuit is known, for example, from the International publication WO 2006/129239 A1. In addition to a plurality of inductive elements, the impedance matching circuit in the document comprises a plurality of MEMS switches as capacitive elements. The capacitance of a MEMS switch can assume two discrete values, and the plurality of connected MEMS switches enables a sufficient tuning range for impedance matching.
One challenge with of known impedance matching circuits for planar antennas is that either the tuning range is too small or the impedance matching circuit has a high level of complexity and a large number of connected elements. The latter results in a relatively high degree of susceptibility to defects.
In one aspect, the present invention specifies an impedance matching circuit with reduced complexity and a reduced number of circuit elements, which circuit nevertheless enables a sufficient tuning range.
Embodiments of the invention comprise a signal path having a node between a signal path input and a signal path output. A first inductive element is connected between the signal path input and the node and a first capacitive element whose capacitance is variably adjustable is connected between the node and the signal path output. A second adjustable-capacitance capacitive element is connected between the signal path input and ground. A second inductive element is connected between the node and ground, and a third inductive element is connected between the signal path output and ground.
Such a connection whose signal path input can be connected, for example, to transmitting or receiving paths of a front-end circuit for mobile radios and whose signal path output is intended to be connected to a planar antenna is a simple, that is to say not very complex, circuit for matching the impedance of the antenna to that of the front-end circuit. The third inductive element may act as an ESD (electrostatic discharge) protective element of the impedance matching circuit and/or the connected front-end circuit. Current pulses which act via the antenna and could damage the front-end circuit or individual components of the latter are then harmlessly discharged to ground via the inductive element.
The first, second and third inductive elements advantageously have Q factors of greater than 15 and the first and second capacitive elements advantageously have Q factors of greater than 10. In this case, the Q factor is a dimensionless measure of the ratio of amplitude to bandwidth of resonance curves or of energy losses in the circuit. In addition, the elements of the impedance matching circuit are advantageously dimensioned in such a manner that the inductances of the first, second and third inductive elements have values of between 0.5 and 22 nH and the capacitances of the first and second capacitive elements can be adjusted in intervals between 0.5 and 12 pF. Such intervals may cover, for example, the capacitance ranges of 0.5 pF to 1.5 pF, of 0.9 pF to 3.2 pF or of 2.6 pF to 8.5 pF.
In one advantageous refinement, the impedance matching circuit comprises a third capacitive element having a Q factor of greater than 50 and a capacitance of between 1 and 35 pF and a fourth inductive element having a Q factor of greater than 15 and an inductance of between 0.5 and 10 nH, which elements are connected in series with one another between the signal path input and ground. Another variation of the impedance matching circuit involves connecting a fourth capacitive element having a Q factor of greater than 50 and a capacitance of between 4 and 18 pF between the signal path input and ground.
The impedance matching circuit is advantageously used in a mobile communication device, the circuit being connected between a receiving path or a transmitting path and a planar antenna, in particular of the PILA type, in such a manner that the signal path input is connected to the transmitting and receiving paths in an electrically conductive manner, and the signal path output is connected to the planar antenna in an electrically conductive manner via an antenna lead having an impedance of between 10 and 60 ohms.
According to one advantageous refinement of the impedance matching circuit, the standing wave ratio in the transmitting path is better (that is to say less) than 3 and the standing wave ratio in the receiving path is better (that is to say less) than 4.
The invention is suitable for matching the impedances of planar antennas in CDMA, W-CDMA, GSM, DVBH, W-LAN, WIFI or other customary data transmission systems in frequency bands between 500 and 4500 MHz.
The tuning ratio of the first or second capacitive element is between 2.5:1 and 3.5:1, that is to say 3:1, for example, in one variant, but is between 3.5:1 and 4.5:1 or between 4.5:1 and 5.5:1 in other advantageous variants and is between 5.5:1 and 6.5:1 in a particularly advantageous variant. In this case, the tuning ratio is respectively defined as the quotient of the largest adjustable capacitance and smallest adjustable capacitance.
At least one of the capacitive elements is preferably a varactor diode whose dielectric layer comprises barium strontium titanate (BST) or whose dielectric layer comprises bismuth zinc niobate (BZN), or alternatively a capacitive element produced using CMOS technology, a connection of MEMS capacitors or a semiconductor varactor diode.
It is preferred to select, as the fourth capacitive element, an element whose Q factor is greater than that of the second capacitive element.
In another advantageous refinement, a fifth capacitive element is connected in parallel with the first capacitive element between the node and the signal path output. In addition, it is preferred to connect a directional coupler to the signal path input in the signal path. A directional coupler makes it possible to determine the fraction of transmitting energy which is actually injected into the antenna from the transmitting signal path. Impedance matching can therefore be reduced to maximizing this fraction. A similar situation applies to reception signals injected into the receiving signal path from the antenna.
It is also very advantageous if the impedance matching circuit comprises a duplexer connected to the signal path input as part of a front-end module.
The inductive and capacitive elements of the matching circuit are preferably in the form of patterned metallizations in a multilayer substrate which may comprise layers of HTCC, LTCC, FR4 or laminate. As a result, a corresponding component also has a space-saving design in addition to its low level of complexity.
In another refinement, an antenna lead having an impedance of between 10 and 60 ohms is connected between the signal path output and a connected planar antenna.
The following list of reference symbols can be used in conjunction with the drawing    AL: Antenna lead    C1: First capacitive element    C2: Second capacitive element    C3: Third capacitive element    C4: Fourth capacitive element    C5: Fifth capacitive element    FE: Front-end module    KP: Node    L1: First inductive element    L2: Second inductive element    L3: Third inductive element    L4: Fourth inductive element    M: Ground    PILA: Planar Inverted L-Antenna    RK: Directional coupler    SP: Signal path    SPA: Signal path output    SPE: Signal path input